The invention relates to composite sheets constituting electromechanical transducers and more particularly those, of these sheets, which have a certain flexibility and extend over a relatively large area, this area being preferably of the order of several square decimeters.
The invention is also directed to transducers equipped with such composite sheets.
It is known that these transducers permit the transformation into variations of an electrical voltage, of the pressure variations which are exerted locally on the sheet and/or the local deformations of this sheet.
The composite sheets concerned comprise essentially for this purpose a dielectric foil "sensitive" to pressure in the sense of electromechanical transduction, which foil is permanently electrically charged or carries a permanent electrical polarisation, interposed between two electrodes or electrically conducting films themselves covered externally with two protective foils, and the transducers equipped with these composite sheets include, in addition to the latter, electrical exploitation means and possibly warning means connected to the two electrodes of said composite sheets.
An interesting application of these transducers, to which the invention relates preferably, but not exclusively, is the detection of apnea, the monitoring of the biologic functions (respiration, cardiac rhythm, . . . ), particularly in nurslings.
Draw-sheets or undersheets are constituted for this purpose, by means of the composite sheets concerned, which are interposed between a mattress and the subject to be watched, lying down, sitting or supported on the mattress: the respiratory movements of this subject cause cyclic local variations in the pressure exerted on this draw-sheet, which variations are manifested by similar cyclic variations of an electrical voltage, and the exploitation means are arranged so as to activate the alarm when said cyclic variations of voltage cease for a time which exceeds a threshold estimated as dangerous or having a form differing excessively from a form considered as normal.
It is a particular object of the invention to improve the fidelity and reliability of transducers of the type concerned.